1. Field of the Invention
The present invention relates to optical communications and optical signal control. In particular, the present invention relates to an optical device and method for delaying photonic signals.
2. Related Art
The fields of communications and data processing are currently transitioning from using electrical signals to using optical signals. As a result, there is an increased need for optical devices that perform various tasks in the control of these optical signals. Such devices include delay blocks for synchronizing optical pulses for communications. The synchronization of optical pulses requires the use of controllable delay devices which do not significantly distort the optical beam as it passes through the device. In particular, robust, compact, lightweight delay devices for use with a variety of predetermined optical frequencies must be developed to be easily integrated into existing optical systems.
One method of creating a low distortion, controllable photonic delay is through the use of xe2x80x9cuniformxe2x80x9d photonic band gap (PBG) structures, also called band-edge delay line devices. Uniform PBG structures typically comprise a stack of alternating layers of refractive materials of similar thicknesses, such as gallium arsenide and aluminum arsenide, which exhibit photonic band gaps in their transmission spectra. These alternating layers have different indices of refraction and can be deposited by well known deposition techniques onto a substrate.
By sending a photonic signal of a given frequency (xcfx89) through this type of delay device, the discontinuity of the indices of refraction imparts a delay to the photonic signal. These devices slow down the photonic signal as a result of scattering inside the uniform PBG structure. Since the photonic delay is proportional to the square of the number of periods contained in the uniform PBG structure, a device can be constructed that imparts a predetermined delay to a photonic signal. The physical processes involved in the photonic signal delay imparted by a uniform PBG structure are described in detail in Scalora, et al., xe2x80x9cUltrashort pulse propagation at the photonic band edge: large tunable group delay with minimal distortion and loss,xe2x80x9d Phys. Rev. E Rapid Comm. 54(2), R1078-R1081 (August 1996), which is incorporated by reference herein.
The present invention generally relates to a device and method of creating an optical signal delay using a compact and readily manufacturable structure. In particular, the present invention interposes a periodicity defect region into a uniform photonic band gap (PBG) structure in order to generate a transmission resonance spike of very narrow bandwidth at or near the center of the photonic band gap of the structure. The introduction of this periodicity defect causes at least an order of magnitude greater photonic signal delay duration than for a uniform PBG device of similar size.
According to one embodiment of the present invention, a Fabry-Perot delay line device is provided for delaying photonic signals of a predetermined frequency and a predetermined bandwidth by a predetermined delay. The Fabry-Perot delay line device includes a first region of periodically alternating layers of refractive materials which exhibit a photonic band gap structure, a second region of periodically alternating layers of refractive materials which also exhibit a photonic band gap structure, and a periodicity defect region interposed between the first and second alternating layer regions. The first region of periodically alternating layers of refractive materials comprises one or more first refractive material layers having a first thickness and a first index of refraction, and one or more second refractive material layers having a second thickness and a second index of refraction. The second region of periodically alternating layers of refractive materials comprises one or more third refractive material layers and one or more fourth refractive material layers. The third refractive material layer has a thickness and an index of refraction similar to or identical to the first refractive material layer of the first alternating layer region. The fourth refractive material layer has a thickness and an index of refraction similar to or identical to the second refractive material layer of the first alternating layer region. The interposed periodicity defect region has a different thickness than either the first or second thickness of the alternating layers of refractive materials in the first and second alternating layer regions. Additionally, the periodicity defect region can have a third index of refraction and a photonic transmission resonance, which are predetermined along with thickness to impart predetermined delay to the photonic signals that pass therethrough.
According to one feature of the present invention, the transmission resonance due to the interposition of this periodicity defect region is located in the center of the photonic band gap. Further, the transmission resonance corresponds to the predetermined frequency of the photonic signal traveling through the Fabry-Perot delay line device.
According to a preferred embodiment of the present invention, the Fabry-Perot delay line device is constructed on a semiconductor substrate using semiconductor materials such as aluminum arsenide (AlAs) and gallium arsenide (GaAs), as the first and second refractive material layers, respectively. Additionally, the periodicity defect region is also a semiconductor material.
According to a second embodiment of the present invention, the Fabry-Perot delay line device includes two or more periodicity defect regions interposed between the periodically alternating layer regions. This embodiment provides for delaying photonic signals of differing predetermined frequencies and predetermined bandwidths by a predetermined range of photonic signal delays.
According to one feature of the second embodiment of the present invention, the first and second photonic transmission resonances are located at different frequency positions in the corresponding photonic band gap of the Fabry-Perot delay line device. Further, the first transmission resonance corresponds to a first predetermined photonic signal frequency, and the second transmission resonance corresponds to a second predetermined photonic signal frequency. Thus, the device may impart different predetermined delays to the first and second predetermined photonic signals traveling therethrough.
According to a third embodiment of the present invention, at unable laser source generates photonic signals traveling through a Fabry-Perot delay line device. The delay imparted by the device varies as a function of the frequency of the photonic signals that pass therethrough.
According to another embodiment of the present invention, Fabry-Perot delay line device also includes electrical contacts located on the periodicity defect regions. A power supply source is coupled to the contact points to impart a variable voltage across the device which varies the indices of refraction of the periodicity defect regions, thereby varying the delay imparted to photonic signals passing therethrough.
Further, according to another embodiment of the present invention, both the band-edge or the Fabry-Perot delay line device can also be constructed using an optical fiber grating. According to this embodiment of the present invention, a fiber grating delay device comprises regions of alternating first and second sections, each section having a different index of refraction, which are periodically spaced along the fiber or waveguide. In addition, a periodicity defect region having a thickness from either the first or second section can be interposed between the regions of alternating sections. Further, piezo-electric or other suitable means can be coupled to the periodicity defect region to alter the optical path length of the periodicity defect region, thereby varying the photonic delay imparted to a photonic signal passing through the fiber grating delay device.
According to another embodiment of the present invention, a defect layer is doped with a gain medium.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.